This invention relates generally to fans having several blades used to move a fluid such as air. In particular, the invention features a fan having blades with variable skew. (i.e., the blade skew varies between at least two of the fan blades.)
Fans are typically constructed with identical blades that are attached at a common hub, the hub being rotated by, e.g., an electric motor through a shaft attached to the hub. The blades are usually evenly spaced around the periphery of the hub. When the inflow velocity of air entering a fan varies (especially circumferential variations), the fan will often generate audible tones at frequencies corresponding to the blade passing frequency (i.e., the frequency at which the blades pass a fixed point) and multiples of the blade passing frequency.
In order to reduce the magnitude of these tones, fans have been constructed with blades located at uneven intervals around the periphery of the hub. (See, e.g., U.S. Pat. No. 3,315,749 to K. W. Parsons et al.) When the blades are unevenly spaced, tones are generated at the same frequency as the frequency at which the shaft rotates, and at multiples of that frequency. Since the shaft rotation frequency is much less than the blade passing frequency, the total number of tones generated within any frequency band is much greater than in the case of evenly spaced blades, and the strength of each tone is correspondingly reduced. If reduced sufficiently, these tones can become inaudible due to the masking effect of various broadband noise sources, including the fan itself. A further advantage of having blades that are unevenly spaced is that the frequency of the lowest frequency tones produced is in a frequency range where the human ear is relatively insensitive. In this way fan noise can be made less objectionable.